The present invention relates to pitch based foam, and more particularly to a pitch based foam which includes a particulate which alters the foam mechanical characteristics.
The extraordinary mechanical properties of commercial carbon fibers are due to the unique graphitic morphology of the extruded filaments. See Edie, D.D., "Pitch and Mesophase Fibers," in Carbon Fibers, Filaments and Composites, Figueiredo (editor), Kluwer Academic Publishers, Boston, pp. 43-72 (1990). Contemporary advanced structural composites exploit these properties by creating a disconnected network of graphitic filaments held together by an appropriate matrix. Carbon foam derived from a pitch precursor can be considered to be an interconnected network of ligaments or struts. As such interconnected networks, they would represent a potential alternative as a reinforcement in structural composite materials.
Recent developments of fiber-reinforced composites has been driven by requirements for improved strength, stiffness, creep resistance, and toughness in structural engineering materials. Carbon fibers have led to significant advancements in these properties in composites of various polymeric, metal, and ceramic matrices.
However, current applications of carbon fibers has evolved from structural reinforcement to thermal management in application ranging from high density electronic modules to communication satellites. This has stimulated research into novel reinforcements and composite processing methods.
High thermal conductivity, low weight, and low coefficient of thermal expansion are the primary concerns in thermal management applications. See Shih, Wei, "Development of Carbon-Carbon Composites for Electronic Thermal Management Applications," IDA Workshop, May 3-5, 1994, supported by AF Wright Laboratory under Contract Number F33615-93-C-2363 and AR Phillips Laboratory Contract Number F29601-93-C-0165 and Engle, G. B., "High Thermal Conductivity C/C Composites for Thermal Management," IDA Workshop, May 3-5, 1994, supported by AF Wright Laboratory under Contract F33615-93-C-2363 and AR Phillips Laboratory Contract Number F29601-93-C-0165. Such applications are striving towards a sandwich type approach in which a low density structural core material (i.e. honeycomb or foam) is sandwiched between a high thermal conductivity facesheet.
Structural cores are limited to low density materials to ensure that the weight limits are not exceeded. Unfortunately, carbon foams and carbon honeycomb materials are the only available materials for use in high temperature applications (&gt;1600.degree. C.). High thermal conductivity carbon honeycomb materials are extremely expensive to manufacture compared to low conductivity honeycombs, therefore, a performance penalty is paid for low cost materials
Typical foaming processes utilize a "blowing" technique to produce a foam of the pitch precursor. The pitch is melted and pressurized, and then the pressure is reduced. Thermodynamically, this produces a "Flash," thereby causing the low molecular weight compounds in the pitch to vaporize (the pitch boils), resulting in a pitch foam. See Hagar, Joseph W. and Max L. Lake, "Novel Hybrid Composites Based on Carbon Foams," Mat. Res. Soc. Symp., Materials Research Society, 270:29-34 (1992), Hagar, Joseph W. and Max L. Lake, "Formulation of a Mathematical Process Model Process Model for the Foaming of a Mesophase Carbon Precursor," Mat. Res. Soc. Symp., Materials Research Society, 270:35-40 (1992), Gibson, L. J. and M. F. Ashby, Cellular Solids: Structures & Properties, Pergamon Press, New York (1988), Gibson, L. J., Mat. Sci. and Eng A110 (1989), Knippenberg and B. Lersmacher, Phillips Tech. Rev., 36(4), (1976), and Bonzom, A., P. Crepaux and E. J. Moutard, U.S. Pat. No. 4,276,246, (1981). Additives can be added to promote, or catalyze, the foaming, such as dissolved gases (like carbon dioxide, or nitrogen), talc powder, freons, or other standard blowing agents used in making polymer foams.
Then, unlike polymer foams, the pitch foam must be oxidatively stabilized by heating in air (or oxygen) for many hours, thereby, cross-linking the structure and "setting" the pitch so it does not melt, and deform the structure, during carbonization. See Hagar, Joseph W. and Max L. Lake, "Formulation of a Mathematical Process Model Process Model for the Foaming of a Mesophase Carbon Precursor, Mat. Res. Soc. Symp., Materials Research Society, 270:35-40 (1992) and White, J. L., and P. M.
Shaeffer, Carbon, 27:697 (1989). This is a time consuming step and can be an expensive step depending on the part size and equipment required.
Next, the "set" or oxidized pitch foam is then carbonized in an inert atmosphere to temperatures as high as 1100.degree. C. Then, a final heat treatment can be performed at temperatures as high as 3000.degree. C. to fully convert the structure to carbon and produce a carbon foam suitable for structural reinforcement. However, these foams as just described exhibit low thermal conductivities.
Other techniques may utilize a polymeric precursor, such as phenolic, urethane, or blends of these with pitch.
See Hagar, Joseph W. and Max L. Lake, "Idealized Strut Geometries for Open-Celled Foams," Mat. Res. Soc. Symp., Materials Research Society, 270:41-46 (1992), Aubert, J. W., (MRS Symposium Proceedings, 207:117-127 (1990), Cowlard, F. C. and J. C. Lewis, J. of Mat. Sci., 2:507-512 (1967) and Noda, T., Inagaki and S. Yamada, J. of Non-Crystalline Solids, 1:285-302, (1969). However, these precursors produce a "glassy" or vitreous carbon which does not exhibit graphitic structure and, thus, has a very low thermal conductivity and low stiffness as well. See Hagar, Joseph W. and Max L. Lake, "Idealized Strut Geometries for Open-Celled Foams," Mat. Res. Soc. Symp., Materials Research Society, 270:41-46 (1992).
One technique developed by the inventor of the present invention, and is fully disclosed in commonly assigned U.S. patent application Ser. No. 08/921,875. It overcomes these limitations, by not requiring a "blowing" or "pressure release" technique to produce the foam. Furthermore, an oxidation stabilization step is not required, as in other methods used to produce pitch-based carbon. This process is less time consuming, and therefore, will be lower in cost and easier to fabricate than the prior art above. More importantly, this process is unique in that it produces carbon foams, such as shown in FIG. 1, with high thermal conductivities, greater than 58 W/m.cndot.K and up to 187 W/m.cndot.K.
Altering the mechanical characteristics, such as the density, compressive strength, and the like, of the carbon foam produced using the inventor's method, however, requires altering the process parameters, such as the temperatures and pressures at various stages of the process. This can affect the thermal conductivity of the final foam product. Therefore, it is desirable to produce a highly thermally conductive foam in which the mechanical characteristics are altered while maintaining the high thermal conductivity of the foam.